cos2C=1-sin^2C=3/5 推出sinC=√10/5
AC/sin45°=AB/sinC=AB/√10/5 AB=2√2
BD=1/2AB=√2
由余弦定理知
sinC=√10/5 cosC=√15/5
sinA=sin(180°-45°-C)=sin√2/2(cosC+sinC)=
CD=√(BD^2+BC^2-2BD*BC*cosB)
=